Instruction manual

Scanning Tunneling Microscopy (STM)
STM Introduction
150 MultiMode SPM Instruction Manual Rev. B
9.1.3 Sample Surface
Samples to be imaged with a scanning tunneling microscope must conduct electricity. In many
cases nonconductive samples can be coated with a thin layer of a conductive material to facilitate
imaging. The sample surface must be conductive enough to allow a few nanoamps of current to
flow from the bias voltage source to the area to be scanned. STMs have been used to scan gold,
silver, platinum, nickel, copper under oil, chrome plating, doped silicon under oil, conducting
polymers, amorphous carbon, blue diamond, diamond-like carbon films, carbon fibers, graphite,
iron-oxide compounds, semi-metals, doped semiconductors (molybdenum disulfide), cobalt-
chromium compounds, stainless steel and liquid crystals. Oxide layers more than a few atoms thick
on the sample tend to affect the scanning and wear down the tip as it is dragged through the oxide.
The feedback loop will extend the tip until a tunneling current flows, even if it must push the tip
through an oxide layer.
For samples that tend to oxidize, tunneling under oil or scanning in a glove box filled with inert gas
can improve the imaging. Silicon oil or paraffin oil (mineral oil) also work well with some samples.
One minor problem involved with the use of oil is the increased difficulty in the coarse positioning
of the tip. The reflection of the tip comes off the liquid instead of the surface of the sample. It is
difficult to tell when the tip is close to the sample surface. The best approach is to lower the tip until
it just touches the surface of the oil, then select Engage.
Note: The STM microscope heads were not designed to operate in UHV.
9.1.4 Vibration isolation
The microscope should be isolated from sources of vibration in the acoustic and subacoustic
frequencies. This requirement can be relaxed somewhat for large-scale images, but atomic-scale
work is extraordinarily sensitive to ordinary room vibrations.
As a final note, the best way to reduce coupling from vibrations is to eliminate as many sources of
vibration as possible. Remember that vibrations can be transmitted to the microscope over the
cable. To reduce this phenomenon, prevent tension in the cable and keep it away from fans and
other noise sources. Also, keep the microscope away from sources of acoustic noise. Loud
conversation can disrupt atomic scale images. Air currents can also disturb atomic images, so it is
best to run the microscope with a cover on.